The present invention relates generally to systems for monitoring conditions at a remote location, particularly status conditions associated with an alarm reporting device stationed at the remote location.
U.S. Pat. Nos. 4,442,320 and 4,528,423 describe a device which can be stationed at a remote location to monitor conditions at the remote location and to provide status reports to a central monitoring facility. The central monitoring facility is provided with a polling unit, such as the unit described in U.S. Pat. No. 4,527,235, to periodically monitor the remotely stationed device and to detect potential deviations which might indicate a need for attention (e.g., an alarm condition). Such devices combine to keep the central monitoring facility advised of the status of the remote location, and to immediately advise the central facility of potential alarm conditions.
To this end, the remotely stationed device operates to receive signals from one or more sensors associated with the location which is being monitored, and to ascertain the condition of these sensors. The polling unit operates to periodically interrogate the remotely stationed device to monitor its status, and accordingly, the status of its associated sensors. Since this exchange is advantageously accomplished by telephone, it is preferable for these periodic interrogations to proceed only when the telephone line which is used to establish communications between the polling unit and the remotely stationed device is not in use (i.e., on-hook) since the signals which are used in performing these periodic interrogations tend to produce unacceptable audible interference with a telephone which is in use. Consequently, during periods when the telephone is in use (i.e., off-hook), such periodic interrogations are discontinued. However, since this would effectively prevent an alarm condition from being reported to the central monitoring facility when the telephone line is otherwise in use, the remotely stationed device is additionally caused to emit a sub-audible tone which continues when conditions are normal, but which is discontinued upon the detection of an alarm condition. This sub-audible tone is used to monitor the status of the remotely stationed device when the telephone is off-hook, instead of performing periodic interrogations. If it is determined that the sub-audible tone has been discontinued, it is presumed that an alarm condition is present, and an immediate interrogation of the remotely stationed device is caused to proceed even though the telephone is off-hook, and in use. This allows normally unaffected usage of the telephone, while assuring that alarm conditions are presently monitored and immediately reported.
The proper operation of such a system is therefore clearly dependent upon an effective determination of the hook status (either on-hook or off-hook) of the communicating telephone line (the subscriber loop), in addition to whether or not the sub-audible tone is present. In connection with conventional telephone systems, the hook status of the subscriber loop was readily determined by monitoring variations in the DC voltage across the subscriber loop. This voltage was known to decrease when the telephone was drawn off-hook, presenting a differential in voltage which could be monitored to make an effective determination of hook status. However, difficulties were encountered in effectively determining the hook status of certain subscriber loops by simply monitoring changes in DC voltage across the loop. For example, due to their impedance, it was found that certain extended subscriber loops tended to exhibit a significantly decreased variation in voltage between on-hook and off-hook conditions. What is more, while this variation would usually take the form of a decrease in potential, increases in potential were also found to occur. The range extension circuitry which is used to accommodate these extended loops was found to even further limit the predictability of these variations in DC voltage. In some cases, this difference in voltage was found to be so small that an effective determination of hook status could not be made by monitoring such DC voltages. Similar difficulties were experienced in connection with some of the more comprehensive subscriber loop configurations which are presently being implemented (e.g., concentrated, multiplexed, etc.).
In such cases, it was found that an effective determination of hook status could not be made simply by monitoring differences in DC voltage across the subscriber loop, in the traditional fashion. Ordinarily, this difficulty could be overcome by monitoring currents within the subscriber loop, since this would be unaffected by the above-described variations in subscriber loop configuration. However, such current measurements are effectively prevented because telephone companies are highly reluctant to allow any components to be placed in series with their subscriber loops, since this could have an adverse effect on the performance of existing telephone equipment, and since available current probes (e.g., hall-effect sensors) which could substitute for this are quite expensive and not entirely reliable in actual application.
It therefore remained to develop a means for effectively determining hook status, while also effectively monitoring the sub-audible tone of an alarm reporting device, without having to rely upon unpredictable variations in voltage or unworkable variations in current.